More from richard clarke, the head of counterterrorism for presidents clinton and bush. Now an abc news consultant. Let's go through some of the clues. The photo showing the bag right by the mailbox before the blast and then disappearing after the blast. Will investigators have a good shot at pinpointing how and why it was placed there? Well, they might if there are pictures of the site when the bag isn't there. What they're going to try to do in their jargon to negate the picture and try to take pictures back and see when the bag was there and when it wasn't so they can get the exact time of placement. Then they'll start looking through the crowd pictures to see who was in the area at the time of placement. And we have tmages of the pressure cooker, as well and that shredded bag that was placed in perhaps and pressure cookers are pretty common as a possible weapon and saw it in the failed bombing in times square, the 2006 bombings in mumbai and promoted on the internet by al qaeda and other terror groups. Yes, so it doesn't tell you much about who did it or whether they're islamist or otherwise, right wing, doesn tell you a thing about motivation, but it does perhaps give you a lead eventually as to where it came from. Remember, the fbi was able to take a 747 that blew up over scotland, pan am 103 into millions of pieces and to put it back together again and to find in all of those pieces the bag where the bomb was placed on that 747 and those pieces w over 100 square miles so the fbi may be able to trace back a pressure cooker if they were able to trace back a piece of luggage in pan am 103. Finally, several who have been briefed by investigators including the ranking republican on the senate intelligence committee saxby chambliss say a lot of what we're learning so far points in the direction of domestic terrorism. Do you agree with that? No, I don't think we have enough information to find out yet. We have to have patience, progress here is not going to be linear. It may take a long time. Your gut tell you this is going to get solved? I read half the terrorist incidents, a third in the u.S. Have ner been solved. This will be solved, george. It may take a while but this will be solved. Okay, dick clarke, thanks very much.

This transcript has been automatically generated and may not be 100% accurate.

Now Playing: Millions of people on severe weather alerts from Michigan to Pennsylvania